Biofuels are fuels of plant origin, which have characteristics similar to fossil fuels, and this allows their use in barely modified engines. These fuels have several environmental advantages. In the case that the biofuels are of plant origin, the balance of carbon dioxide in its combustion is neutral since it can be considered that the same amount of carbon dioxide produced in said combustion, has been previously consumed from the carbon dioxide from the atmosphere through photosynthesis cycles (over a period of years). In addition, biofuels do not contain the element nitrogen or the element sulfur. For this reason, the oxides of these elements will not be produced in its combustion, thus preventing the formation of nitrous gases responsible for skin irritation and damage to the respiratory system and the origin of the tropospheric ozone and smog formation. It is known that these oxides promote the acid rain formation, being sulphur oxides the main cause of the same.
The first-generation of transport biofuels forms mainly biodiesel (along with bio-ethanol). Today, methyl and ethyl esters of fatty acids referred to as biodiesel (or FAMEs). Biodiesel is obtained by transesterification of vegetable oils with methanol or ethanol. This biofuel has some disadvantages. Since it is not a hydrocarbon, it is not interchangeable with the current diesel. This means that engines and/or vehicles need an adaptation to be able to use 100% biodiesel as fuel. At present these adaptations can already be technically carried out but to avoid the economic cost entailed by a complete change, biodiesel is only added up to a 5% to conventional diesel. Another drawback of biodiesel is that an extended or inappropriate storage can promote its decomposition and the release of fatty acids. These acids are not completely soluble in the mix and the formation of solids may cause problems in ducts and filters, as well as the possible corrosion caused by its acidic properties. However, the main reason why biodiesel cannot replace conventional diesel in the future is the origin of the former. Vegetable oil is obtained mainly from crop plants which makes it compete for arable land. This means that at the end the biodiesel production competes with food production, causing a significantly increase in the price of some basic foodstuffs.
To avoid competition with food production a second generation of biofuels has been developed, which must avoid plants, turnips, seeds, tubers, etc. having direct use as food and, in general, any plant biomass requiring arable land. On this basis it is intended to develop second generation biofuels from cellulose or hemicellulose that may come from wood (wood chips or sawdust) but also from any kind of plant biomass waste.
Possible solutions to the problem of the second-generation biofuels production have been recently suggested. In the process described by J. A. Dumesic et al. (Science 2005, 308, 1446-1450; PTC Int. Appl. WO2008151178, 2008; US Patent 20090124839, 2007) the aldol condensation of 5-hydroxymethylfurfural (HMF; or furfural) is carried out to produce molecules with 9, 12 or 15 carbon atoms (see scheme 1) that in subsequent steps can be hydrogenated to their corresponding alkanes. This technology has several drawbacks. For example the fact that the aldol condensation needs a second starting material, since an aldol condensation of the HMF or furfural with itself is not possible, whereby it is necessary to carry out a crossed aldol condensation. To this end, Dumesic and collaborators use acetone as a connector of two furanic molecules. However, a crossed aldol condensation involves, because of its nature, a lower selectivity, since acetone can, in fact, condense with itself.

This has as a consequence that if stoichiometric ratios are used, which means 2 moles of furfural and 1 mole of acetone (since acetone can react by both ends), between 16% and 37% of components with only 5 carbon atoms would be obtained, having a very limited interest as components for gasoline (Appl. Catal. B Environ. 2006, 66, 111-118). A second product with 8 carbon atoms which tends to be one-third of the mixture appears under other conditions. This condensation product is hydrogenated to n-octane which does not have an interesting application in gasoline since it has linear chain, nor in diesel because of the low molecular weight. To increase selectivity to 85% with a 71% yield, the condensation has to be carried out in an aqueous phase and hydrogenation in hexadecane as a solvent at 120° C. thereby making the process more expensive (Appl. Catal. B Environ. 2006, 66, 111-118). The authors themselves realized the disadvantages caused by selectivity and proposed as an alternative the hydrogenation of the furan ring to tetrahydrofuran since these derivatives are capable of carrying out an aldol condensation with themselves and this would ensure a high selectivity. However, chemoselective hydrogenation of, for example, furfural to tetrahydrofurfural in one step is still a challenge and it is currently carried out in several stages. In any case, if a multistage process is accepted, molecules with a total of 10 carbon atoms can be obtained (Science 2005, 308, 1446-1450) as well as by the formation of furoin.
An alternative solution for the second-generation biofuels production is described in R. D. Cortright, WO2008109877, 2007; Int. Sugar J. 2008, 110, 672-679, producing in a first step mixtures of compounds with 4 carbon atoms or more from compounds oxygenated in aqueous solution in the presence of a deoxygenation catalyst and a condensation catalyst (Aqueous Phase Reforming). In order to obtain high levels of alkanes the inventors use basic catalysts to condense ketones and aldehydes such as in the case of Dumesic or the oligomerization of alkenes. However their way to combine molecules with low number of carbons is not enough to give molecules with a sufficient number of carbon atoms to be used as Diesel. Thus, the content in raw products of molecules with ten carbon atoms or more is below 50%. FIG. 1 shows the Cortright process, adapted from Int. Sugar J. 2008, 110, 672-679.
In other attempts to convert biomass into fuels, oxygenated products are obtained. These do not meet the requirement demanded for the second-generation biofuels such that they can be used in engines currently in use and could, perhaps, be used as additives which can only be added to fuel in limited concentrations. Examples of these can be 2,5-dimethylfuran (Nature, 2007, 447, 982-986), or ethers or esters of 5-hydroxymethylfurfural (PCT Int. Appl. WO2009030510, 2007).
Dumesic (Angew. Chem. Int. Ed. 2007, 46, 7164-7183), in addition to the process explained above with the key step of aldol condensation, describes other processes such as dehydration and hydrogenation of sorbitol or xylitol to light linear alkanes. However, this latter process cannot be considered as an alternative to produce hydrocarbons increasing the number of carbon atoms to more than the initial five or six (see also Angew. Chem. Int. Ed. 2004, 43, 1549-1551).
The present invention refers to a procedure for transforming the biomass-derived products in good quality diesel.